Re: [ATM] Barlow quality/testing

["Scott Ewart" <scotte@frontiernet.net>]

Tony,
    Let me tell you a little story.  I've posted this in the past, so 
you can look it up in the archives.
    After figuring my first mirror, a 12.4" f/5.5, I decided to 
re-figure my 13.1" f/4.4 Coulter mirror.  I had some difficulty, but 
when I got it so the test numbers looked good, I set up the scope for 
a star test.  It looked good with my 5mm Nagler, but overcorrect with 
the 1.8x Tele Vue barlow.  I kept going back and forth between them 
and decided I must be seeing more with the barlow.  So I kept 
polishing till I got back to good (about the same) test numbers two 
weeks later.
    The star tests looked the same, so I really started to scratch my 
head.  I started testing with lots of different combinations and with 
other scopes.  What I discovered was that the 1.8x barlow that showed 
NO change in spherical aberration with an f/5.5 or f/6 DID show 
overcorrection with an f/4.4.  I didn't even know this was possible.
    I got in touch with Al Nagler and he told me he had been aware of 
this and had long since discontinued that barlow in favor of the 2x 
version which did not have this problem.  So I believe everything he 
makes now is free of such problems.  I know my newer 2x barlow is dead 
on.

Scott


----- Original Message ----- 
From: "tony gondola" <acgna@comcast.net>
To: <atm@atmlist.net>
Sent: Friday, May 12, 2006 11:47 AM
Subject: [ATM] Barlow quality/testing


| Does anyone have any insight into the overall quality and general 
aberrations of typical barlow lenses? As an avid lunar imager the 
barlow is a critical link in the imaging chain and I'm not convinced 
at this point that most of what's commercially available is really the 
best answer. I'm especially gun shy when a manufacturer claims to 
reduce aberrations when used with certain ocular designs. For CCD work 
that last thing you'd want is a barlow that's introducing aberrations 
of an a sign opposite that of the typical ocular in an effort to 
obtain better visual performance. What the imager needs is a pure 
barlow with good color correction and no introduction of spherical 
aberration. I've star tested several and none so far can really meet 
that standard. Any thoughts out there on this? I hate the idea of 
sweating blood to make a great planetary mirror only to be undone by 
the barlow.
|
| Tony
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